a) Field of the Invention
The present invention relates to an objective lens system for endoscopes.
b) Description of the Prior Art
There is conventionally known the retrofocus type objective lens system for endoscopes illustrated in FIG. 1, as exemplified by the objective lens system disclosed by Japanese Patent Kokai Publication No. Sho 49-121547. This retrofocus type objective lens system comprises a first lens unit I arranged on the object side of an aperture stop S and a lens unit II arranged on the image side of the aperture stop S. This objective lens system is designed so as to have a wide field angle by strongly refracting the principal ray p by the lens unit I which is arranged before the aperture stop S and has a negative refractive power, and allow light bundle to be incident perpendicularly on an end surface of an image guide G by arranging the aperture stop at the front focal point of the lens unit II which has a positive refractive power so as to compose the telecentric optical system wherein the principal ray is parallel with the optical axis. Loss of light amount is reduced by allowing the light bundle perpendicularly on the end surface of the image guide as described above.
Further, there are widely used, in these days, video endoscopes which employ solid-state image pickup devices in place of the image guides.
Image pickup systems for video endoscopes are classified into the field sequential color type and the dot sequential color type. The dot sequential color type image pickup system includes color encoding filters (generally referred to as color mosaic filters) which are fine color filters integrated in mosaic patterns at locations corresponding to picture elements of a solid-state image pickup devide on the incidence side of the solid-state image pickup device so as to obtain color signals.
When the principal ray is incident obliquely on the imaging surface in a video endoscope incorporating the dot sequential color system, the ray which is to fall at a large angle of incidence on the solid-state image pickup device falls, after passing through the color filter, not on the picture element on which the ray should originally be incident but on the picture element adjacent thereto, thereby producing color ununiformity (color shading) on an image. It is impossible to prevent the production of the color shading, for example, in the optical system disclosed by Japanese Patent Kokai Publication No. Sho 62-173415 in which the principal ray is incident obliquely on the image pickup device as shown in FIG. 2.
For the reason described above, it is necessary to use telecentric objective lens systems also in video endoscopes.
However, it is difficult to correct coma in a telecentric objective lens system like the objective lens systems shown in FIG. 1, in which an aperture stop S is arranged right after a lens unit I having a negative refractive power. In this objective lens system, the coma produced by the lens unit I are connected by lens surfaces arranged on the image side of the aperture stop. Further, in the objective lens system shown in FIG. 1, the lens surfaces arranged close to the aperture stop, like the image side surface of the second lens component (the lens component arranged right after the aperture stop) have centers of curvature nearly aligned with the center of the aperture stop, whereby these surfaces produce little coma and cannot correct the asymmetry of the coma produced by the first lens component (the negative lens component) even when radii of curvature are shortened on these surfaces.
Since the coma is corrected insufficiently by the second lens component as described above, height of ray is enhanced by locating the third lens component far from the aperture stop and coma is corrected by shortening the radius of corvature on the cemented surface in the objective lens system shown in FIG. 1. When the power of the first lens component is strengthened for widening the field angle of the objective lens system shown in FIG. 1, remarkable coma is produced by the first lens component. When this coma is corrected, the second and later lens components produce remarkable spherical aberration and astigmatism. In order to correct these aberrations favorably, it is necessary to reserve wide airspaces between the lens components and weaken the powers of the individual lens components. The total length of the objective lens system will be prolonged and the outside diameters of the lens components will be enlarged accordingly.
Since the video endoscope comprises the solid-state image pickup device which has sensitivity not only to the visible light but also to the infrared light, it hinders colors from being reproduced correctly on images projected onto a monitor TV. For correct color reproduction on images projected onto the monitor TV, it is necessary to use a filter which cuts off the infrared light in the video endoscope.
Further, when medical treatments are carried out by using a laser light having a wavelength within the near infrared region or the far infrared region, the solid-state image pickup device is saturated by the laser light, thereby hindering clear observation of the locations to be treated. In order to solve this problem, a filter which cuts off the laser light to be used for medical treatment must be arranged in the optical system of the video endoscope.
When an attempt is made to reserve a space for arranging these filters before the aperture stop in the lens systems illustrated in FIG. 3, the distance as measured from the first lens component to the aperture stop is prolonged and the outside diameter of the first lens component is enlarged. When another attempt is made to shorten the outside diameter of the first lens component, the inclination angle of the offaxial principal ray to pass through the aperture stop (the angle T shown in FIG. 1) will be reduced. In case of the telecentric optical system in which the front focal point of the lens unit II arranged after the aperture stop is located at the position of the aperture stop, the reduction of the inclination angle T shown in FIG. 1 will prolong the distance as measured from the aperture stop to the lens unit II, thereby prolonging the total length of the optical system. Further in order to reserve the space for arranging the above-mentioned filters after the aperture stop, the lens unit II must have a longer focal length, thereby obliging to prolong the total length of the optical system. For this reason, the optical system shown in FIG. 1 is not suited for use in the video endoscope since it prolongs the total lengths of the video endescopes. Furthermore, remarkable distortion is produced in the objective lens system for endoscopes which satisfies the telecentric condition.
Distortion is dependent on angle of incidence .theta..sub.1 of the principal ray on the entrance pupil. Further, image height is a function of the angle of incidence .theta..sub.1.
When distortion is represented by D(.theta..sub.1) and image height is designated by H(.theta..sub.1) distortion D(.theta..sub.1) (is defined by the following formula (i): EQU D(.theta..sub.1)=100.times.[{H(.theta..sub.1)/f.multidot.tan.theta..sub.1 {-1] (%) (i)
wherein the reference symbol f represents the focal length of the telecentric objective lens system.
Using A(.theta..sub.1) which is a function of .theta..sub.1, image height H(.theta..sub.1) is generally expressed as follows: EQU H(.theta..sub.1)=fA(.theta..sub.1)
When this formula is used in the formula (i), we obtain: EQU D(.theta..sub.1)=100.times.[(A(.theta..sub.1)/tan.theta..sub.1)-1] (%) (ii)
As is understood from the formula (ii), the relationship between the distortion and the angle of incidence of the principal ray is determined by the function A(.theta..sub.1) only which defines the relationship between the image height and the angle of incidence of the principal ray. That is to say, the function A(.theta..sub.1) represents a characteristics of the distortion in the optical system.
Generally speaking, the function A(.theta..sub.1) is dependent solely on imaging relationship of pupil, and classified into dependency on the paraxial pupil magnification and imaging aberrations of pupil (offense against the sine condition determined by tracing rays from points on the pupils taken as object points and spherical aberration). When it is assumed that an objective lens system is free from aberrations of pupil, i.e., when the sine condition of pupil is satisfied at all image heiqhts in an obJective lens system and spherical aberration is not produced neither at the entrance pupil nor the exit pupil, A(.theta..sub.1) is determined uniquely by using only the paraxial pupil magnification as a parameter. Speaking concretely, A(.theta..sub.1) is expressed as follows by using the paraxial pupil magnification which is represented by .beta..sub.g : ##EQU1##
In order to maintain the telecentric condition, it is necessary to reserve a sufficiently large absolute value of the paraxial pupil magnification .vertline..beta..sub.E .vertline. for an objective lens system for endoscopes.
When .vertline..beta..sub.E .vertline. is sufficiently large, the formula (iii) allows A(.theta..sub.1) to be approximated to sin.theta..sub.1 and the distortion is expressed as follows: EQU D(.theta..sub.1).about.100.times.[cos.theta..sub.1 -1](%) (iv)
As is understood from this formula (iv), negative distortion increases as .theta..sub.1 is enlarged
In the objective lens system which satisfies the telecentric condition, the distortion is produced due to the paraxial relationship of pupil. In order to correct this destortion, it is necessary to produce offense against the sine condition in imaging of pupil. In an attempt to correct the distortion in an optical system for endoscopes, an increase of the offense against the sine condition in imaging of pupil will produce direct influences on the asymmetrical aberrations in imaging of an object, i.e., astigmatism and coma. An attempt to design an objective lens system compact and widen the field angle thereof also makes it difficult to correct aberrations, especially the offaxial aberrations. In designing an objective optical system for endoscopes, it is therefore important how to favorably correct the aberrations other than the distortion after the optical system has a wide field angle, sufficiently corrected distortion and a small outside diameter or a compact design. It is impossible to obtain a preferable objective lens system for endoscopes without satisfying all of these requirements at the same time.
As an conventional example of objective lens systems for endoscopes which have distortion and the other abennation corrected favorably by using aspherical surfaces, there is known the lens system disclosed by Japanese Patent Kokai Publication No. Sho 57-173810. However, this objective lens system has a narrow field angle of 56.degree. and distortion which is not corrected completely.
Further, the optical system disclosed by Japanese Patent Kokai Publication No. Sho 60-169818, which is illustrated in FIG. 4, is a telecentric optical system composed of a front lens unit having a negative power and a rear lens unit having a positive power which are arranged on both sides of an aperture stop, and has distortion corrected by using at least one aspherical surface in the front lens unit. However, the optical systems preferred as the embodiments of the above-mentioned Japanese Patent Kokai Publication No. Sho 60-169818 are not compact and undesirably comprise very large numbers of lens elements.
Furthermore, the optical system disclosed by Japanese Patent Kokai Publication No. Sho 61-162021, shown in FIG. 5, is a telecentric optical system which uses at least one aspherical surface on each of the object side and the image side of an aperture stop, and has corrected distortion. However, this optical system is not compact, like the optical system proposed by Japanese Patent Kokai Publication No. Sho 60-169818, and not practical since the aspherical surfaces used therein have large departures from the reference spheres and can hardly be manufactured in practice.